DUMPING GAS FROM TAIL PIPE

i have a runner which I have been driving since I got it from a freind about 3 months ago it is still moving but I noticed plugs 2 and 4 are not burning gas. When I removed them they have a lot of carbon, I changed all 4 but still no improvement it still dumps a lot of gas from the tail pipe. I have driven over what I think are long drives eg 4 to 6 hour destinations prior to the fault